lable at ScienceDirect

Dyes and Pigments 116 (2015) 106e113

Contents lists avai

Dyes and Pigments

journal homepage: www.elsevier .com/locate/dyepig

Pigments based on Cr and Sb doped TiO2 prepared by microemulsion-mediated solvothermal synthesis for inkjet printing on ceramics

Marc Jovaní a, María Domingo a, Thales R. Machado a, b, Elson Longo c,H�ector Beltr�an-Mir a, *, Eloisa Cordoncillo a

a Departamento de Química Inorg�anica y Org�anica, Universitat Jaume I de Castell�on, Avda. Sos Baynat s/n, 12071 Castell�on de la Plana, Spainb LIEC-Laborat�orio Interdisciplinar de Eletroquímica e Ceramica, Departamento de Química, UFSCar-Universidade Federal de S~ao Carlos, Rod. WashingtonLuis km 235, P.O. Box 676, 13565-905 S~ao Carlos, S~ao Paulo, Brazilc LIEC-Laborat�orio Interdisciplinar de Eletroquímica e Ceramica, Instituto de Química, UNESP-Universidade Estadual Paulista Júlio de Mesquita Filho, P.O.Box 355, 14801-907 Araraquara, S~ao Paulo, Brazil

a r t i c l e i n f o

Article history:Received 31 July 2014Received in revised form22 December 2014Accepted 15 January 2015Available online 23 January 2015

Keywords:PigmentTitanium dioxideOrange ceramic pigmentInkjetCeramicsSolvothermal synthesis

* Corresponding author. Tel.: þ34 964 728245; faxE-mail address: mir@uji.es (H. Beltr�an-Mir).

http://dx.doi.org/10.1016/j.dyepig.2015.01.0160143-7208/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

A ceramic pigment with nominal composition Ti0.97Cr0.015Sb0.015O2 was prepared by microemulsion-mediated solvothermal method at 180 �C. Anatase or rutile single phase was obtained depending onthe synthesis conditions. Scanning electron microscope analysis showed the formation of spheres withparticle size around 600 nm. The anatase to rutile transformation temperature was determined by XRDand Raman spectroscopy. The evolution of the colour was studied, and it was related with the poly-morphic transition. Yellow pigments were obtained at low temperature and huge orange colour wasobserved at high temperature. Powders prepared at 180 �C were tested with an industrial frit. Similarchromatic coordinates of an industrial orange ceramic pigment obtained at high temperatures wereobserved. z-potential values of the particles were ~�57 mV. The size, shape, colour and electrostaticstability of these particles make them potential candidates to be applied in glazes or inkjet printers asorange ceramic pigments.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Titanium dioxide, TiO2, has a wide range of applications. Sinceits commercial production in the early twentieth century, it hasbeen widely used as a pigment [1], sunscreens [2,3], paints [4],toothpaste [5], etc. Titanium dioxide occurs mainly in three crys-talline forms: rutile, anatase and brookite. Rutile is the stable phase,and anatase and brookite are metastable. Both anatase, space groupI4/amd, and rutile, space group P42/mnm, are tetragonal. Structuresconsist of TiO6 octahedra, sharing four edges in anatase and twoedges in rutile [6,7].

Nowadays, the field of nanotechnology has generated a greatdeal of interest becausematerials have numerous new properties innanosize-scaled. These size-dependent properties include newphase transition behaviour, different thermal and mechanicalproperties, interesting surface activity and reactivity, and unusualoptical, electrical and magnetic characteristics [8e10]. In this way,

: þ34 964 728214.

and more closely in the field of pigments, TiO2 have a massivepotential market. Study of the particle size of TiO2 is essentialbecause it affects phase and thermal stability as well as surface andbulk properties [11e13]. The knowledge of the particle size iscrucial to predict the behaviour of the nanoparticles in industrialprocesses, like in the case of ceramic decoration which the use ofhigh temperatures are required [14].

Ceramic submicronic pigments have been developed for inkjetdecoration of ceramic tiles using quadrichromic technology, being anew field of application. These pigments with particle size less than1 mm are able to overcome some actual problems of the inkjet in-dustrial processes [15]. The use of pigmenting particles at the nano-scale is necessary for inkjet applications [16,17]. At present in theindustry, these particles are basically obtaining by different millingsteps, producing submicronic particles with no round shape [18,19].The use of submicronic pigments in inkjet technology can solvedproblems like nozzle clogging, dispersion or instability caused bymicronized pigments, and moreover, remove the milling stages [14].

Wet chemistry methods are one of the best options to prepareTiO2 nanoparticles. The literature reports approaches for the

M. Jovaní et al. / Dyes and Pigments 116 (2015) 106e113 107

synthesis of nanoparticles of titania, including thermal hydrolysis[20,21], solegel [22,23], hydro/solvothermal method [24,25] andmicroemulsion processes [26,27]. Among them, solvothermal andmicroemulsionmethods are extensively used for the preparation ofnanomaterials.

Solvothermal method has many advantages such as: (a) the finalproduct can be obtained directly at relatively lower reaction tem-perature, (b) crystalline products with different composition,structure, and morphology could be prepared modifying the syn-thesis conditions like temperature, pH, times or reactant concen-tration, and (c) it produces high purity particles compare withtraditional solidesolid routes.

Therefore, the solvothermal synthesis is a good method for thepreparation of oxide ceramic fine powders [24]. However, largesizes of TiO2 nanocrystals and poor dispersion stability usuallyappear in the materials prepared by solvothermal methods.

Microemulsions is a powerful method for obtaining ultrafineand nanometric particles with controlled size and shape [28]. TiO2nanomaterials prepared by micelle method have often amorphousstructure, and calcination is usually necessary in order to inducehigh crystallinity [10].

Based on the advantages of each method, a combination ofmicroemulsion with solvothermal method has been explored toprepare nanomaterials such as SrCO3 nanostructures [29] orCa10(PO4)6(OH)2 [30]. Microemulsions with nanosized aqueouscores have been used as the reaction media for preparation of TiO2nanomaterials [31,32]. Shen et al. [33] have successfully synthesizedrutile and anatase with microemulsion-mediated solvothermalmethod. They studied the preparation of rutile or anatasemodifyingthe synthesis conditions such as the amount of urea in aqueousphase of themicroemulsion. The effect of pHonTiO2 phase structurehave been also studied [12,34]. Regarding these studies, the low pHfavours the formation of rutile phase while more alkaline mediafavours anatase phase formation. Other studies were done in orderto control the growth of the particles. Somiya et al. [35] preparednanomaterials by hydrothermal microemulsion process in order toprevent grain growth of the nanoparticles. They conclude that theaqueous micelles in microemulsions act as microreactors to confinethe growth of TiO2 powders.

As it mention before, TiO2 has been widely used as a pigment. Inthe industry,where thesolid solutionused isTi0.97Cr0.015Sb0.015O2, thepigment is manufactured starting from anatase with chromium (III)oxide as chromophore element and antimony (III) oxide as counter-ions in presence of several mineralizers. In this case, the colour isacquired by calcinations at high temperatures around 1200 �C,wherethe development of the colour occurs during the anatase-rutiletransformation [36]. Anatase to rutile transformation is reconstruc-tive, therefore, transformation involves breaking and reformingbonds [7]. This reconstructive transformation involves a contractionof the c-axis and involving a volume contraction around 8% [37].

In the literature, synthesis of CreSbeTiO2 pigment was carriedout by different preparation routes. C. Gargoni et al.[38] preparedsamples of composition Ti0.97Cr0.015Sb0.015O2 with rutile phase byseveral routes: ceramic method, heterogeneous ammonia copre-cipitation method, homogeneous urea coprecipitation method,Pechini polyester method, and aqueous-organic coprecipitationmethod in a water-ditehylenglycol. Authors concluded that finalcolour in a conventional glaze from powders obtained by non-conventional methods was worse than colour obtained frompowders prepared by the ceramic method. Other authors [14,39]used the polyol non-conventional method to synthesize nano-particles of this pigment. Stable ceramic inks were prepared with ananometric size by this method.

In this work, Cr,Sb-doped TiO2 ceramic pigments were preparedby microemulsion-mediated solvothermal method. As far as we

know, synthesis of this pigment has not been yet reported usingthis method. This route allows an accurate control of the phase andthe particle size of the pigment, and therefore, it is possible todetermine the influence of these properties to the final colour athigh temperature in ceramic decoration. In spite of this methodhave been used to prepare TiO2, the incorporation of the Cr and Sbto this structure have not been reported using this method. In theliterature [33] and, as well as in this study, the phase (anatase orrutile) was controlled by the pH of the aqueous phase. However,time of the solvothermal treatment has been studied in this workdue to the presence of the chromophores ions. These ions must beincorporated in the structure to form a solid solution, and therefore,they can also affect to the final phase present and the kineticconditions of the reaction. Due to it, the time of the reaction is animportant parameter to consider here. Studies of the rutile-anatasetransformation were also conducted in this work.

Microemulsion-mediated solvothermal method has been ableto synthesise pigments with optimal colouration, controlling thephase, shape and size of the particles to be applied in inkjet printingof ceramics. For this application, the inorganic pigment was ob-tained at high temperature by the traditional solidesolid reaction.However, in this method, calcinations were unnecessary becausethe solvothermal treatment promoting the crystallization of thephase at low temperatures. Therefore, these Cr,Sb-doped TiO2submicroparticles obtained at low temperatures (180 �C) wouldhave potential applications in the field of ceramic inks.

2. Experimental

Samples of Ti0.97Cr0.015Sb0.015O2 solid solution were prepared bya microemulsion-mediated solvothermal route. The synthesisprocedure was as follows: first, 10 mL of Triton X-100 (surfactant),3 mL of n-hexanol (cosurfactant 98%) and 16 mL of cyclohexane(SigmaeAldrich, �99.5%) were mixed under magnetic stirring,making up the oil phase. Second, 2 mL of TiCl4 (Fluka, �99%) so-lution, previously prepared in an acid medium of HCl 4M, 2 mL ofH2O and the specifics amounts of CrCl3$6H2O (Probus, 93%) andSbCl3 (SigmaeAldrich, �99%), together with the necessary amountof urea (Fluka, �99.5%) to obtain the desired phase, were mixedunder magnetic stirring (aqueous phase). The amount of urea wasfixed to 4.5 g and 1.5 g per gram of pigment to obtain anatase (A) orrutile (R) single phase, respectively [33].

Then, the aqueous phase was added dropwise to the oil phaseunder stirring mediated a peristaltic pump at room temperature,forming a clear microemulsion. The microemulsion was mixed un-der magnetic stirring for 48 h and, then, placed in a Teflon-linedstainless steel autoclave and heated at 180 �C in an oven for vari-able times. The precipitate was collected by centrifugation andwashed repeatedly with ethanol. After this process, samples weredried in air at room temperature. A scheme of the general prepa-ration of the samples is shown in Fig. 1, and the different treatmentconditions used in each case (time and the amount of urea) areshown in Table 1.

It is well known that the anatase-rutile phase transition involvesa volume contraction, and it depends on variables such as size,morphology, etc. [7]. Therefore, in order to study the anatase-rutilephase transition in these samples, the powder with anatase phaseobtained at 180 �Cwas annealed at different temperatures between750 and 1080 �C for 2 h and cooled slowly inside the furnace.

Powders of samples at 180 �C, where single phase and optimalcolour were obtained, were mixed with one industrial frit (4% inweight of the pigment) using water as a dispersing medium. Then,the dispersion was applied to white twice-fire bodies, to verifycomposition stability as a ceramic colourant. A commercial trans-parent frit was chosen. The frit composition used is given in Table 2.

Table 2Frit composition.

Composition (wt%)a

SiO2 Al2O3 ROb R2Ob ZnO ZrO2 Temperature/�C

67 13 9.4 10 0.4 0.2 1080

a The percentages do not represent quantitative analyses.b R ¼ alkaline or alkaline earth metals.

Fig. 1. Scheme of the TiO2 sample preparation.

M. Jovaní et al. / Dyes and Pigments 116 (2015) 106e113108

After drying, the pieces were fired in an electric kiln. The heattreatment applied, corresponds to a standard firing cycle used in aceramic tile industry where the highest temperature of the cyclewas 1080 �C for 5 min. This cycle involve five steps: ramping to800 �C in 18min, heating from 800 �C to glaze firing temperature in17 min, 5 min hold at 1080 �C, cooling to 600 �C in 20 min, andfinally cooling to room temperature in 15 min.

2.1. Characterization

Phase analysis of the samples was performed by powder XRDusing a Bruker D4 Endeavour diffractometer with CuKa radiation.Data were collected by step-scanning from 2q ¼ 20 to 70� with astep size of 0.05� and 1.5 s of counting time at each step.

Scanning electron micrographs of the samples were taken on afield emission scanning electron microscope (SEM) JEOL 7001F,equipped with a spectrometer of energy dispersion of X-ray (EDX)from Oxford instruments, using acceleration voltage ¼ 15 kV.Samples for microstructures and microanalysis determinationswere deposited in an aluminium holder and sputtered by platinum.Dynamic Light Scattering (DLS, ZetaSizer-NanoSeries Malvern In-struments, Malvern, UK) was also used to measure the z-potentialof the as-prepared powder samples.

Raman spectra were recorder on a RFS/100/S Bruker Fouriertransform (FT-Raman) spectrometer, with a Nd:YAG laser excitationlight at 1064 nm in a spectral resolution of 4 cm�1, in order toconfirm the polymorphic phase (anatase or rutile).

UVeVisible diffuse reflectance spectroscopy and colourimetricstudy of the glazed fired samples were performed on a CARY 500SCAN VARIAN spectrophotometer in the 200e800 nm range. BaSO4

Table 1Different treatment conditions to obtain rutile (R) or anatase (A) phase.

Ref. Urea(g)/Pigment(g) Time (h)

R1 1.5 17R2 1.5 24R3 1.5 48A1 4.5 17A2 4.5 24A3 4.5 48

was used as a reference. Reflectance (R∞) was converted to absor-bance (K/S) by the Kubelka-Munk equation: K/S¼ 2(1� R∞) x 2 R∞

�1

[40]. The positions of the main absorbance peaks in the opticalspectra were determined through a deconvolution procedure thatallowed obtaining more accurately values for the electronic transi-tions. The CIELab colour parameters L*, a*, and b* of the glazed firedcompositions and the anatase powders at different temperatureswere determined by coupling an analytical software for colourmeasurements to the Varian spectrophotometer, using a standardilluminant D65, to differentiate the pigment in terms of colour. L* isthe lightness axis [black (0) towhite (100)], a* is the green (<0) to red(>0) axis, and b* is the blue (<0) to yellow (>0) axis.

3. Results and discussion

3.1. Optimization of the crystalline phase and microstructuralcharacterization

Microemulsions were heated at 180 �C for variable times tooptimize the preparation of a rutile or anatase single phase. Several

Fig. 2. XRD patterns of TiO2 synthesised with different amounts of urea and differenttimes of reaction at 180 �C in order to prepare rutile (a) or anatase (b) phases.

Fig. 3. Raman spectra for TiO2 samples prepared with (a) 1.5 g of urea and after 24 h ofreaction, R2, and (b) 4.5 g of urea and after 17 h of reaction, A1. Rutile or anatase phaseswere observed in (a) and (b), respectively.

Fig. 4. SEM images of A1 (a) and R2 (b) powder sam

Fig. 5. Raman spectra of A1 sample fired at different temperatures.

M. Jovaní et al. / Dyes and Pigments 116 (2015) 106e113 109

factors such as temperature, time and pH needs to be considered toobtain single phases [12,32,34]. The aqueous phase with TiCl4 so-lution in HCl has a very low pH, and therefore, the amount of ureaplays an important role controlling the pH of this aqueous phase.

XRD patterns of TiO2 synthesized at 180 �C with different timesto obtain rutile or anatase are shown in Fig. 2(a) and (b),respectively. The crystalline phase evolution of TiO2 was observedin both cases. Single phase of rutile [JCPDS 21-1276] was obtainedwhen the treatment time was higher than 17 h and the amount ofurea was fixed to 1.5 g, Fig. 2(a). Secondary phase of anatase[JCPDS 21-1272] was identified at short times of reaction. There-fore, the reaction needs more than 17 h for the formation of rutilesingle phase. When the amount of urea was adjusted to 4.5 g,anatase single phase was observed by XRD at short solvothermaltreatment times, Fig. 2(b). Secondary phase of rutile appeared

ples with the average particle size in each case.

Table 3Chromatic coordinates of A1 powder samples at different temperatures.

L* a* b*

180 �C (A1) 93 �5 21750 �C 78 0 23800 �C 76 1 23850 �C 77 3 28900 �C 77 5 311000 �C 72 8 311080 �C 70 14 36

Fig. 6. CIELab chromatic coordinates of the A1 powder fired at different temperatures.

M. Jovaní et al. / Dyes and Pigments 116 (2015) 106e113110

when the reaction time was increasing. R3 and R2 samples consistof single phase of rutile, while A1 sample is 100% anatase. R2 andA1 samples were chosen to continue the study due to bothsamples presents single phase by XRD and the shortest reactiontimes.

In order to confirm results obtained by XRD, TiO2 powders werefurther characterized by Raman spectroscopy. Fig. 3 shows theRaman spectra of R2 and A1, showing the characteristic bands of

Fig. 7. Micrographs of the A1 sample at different temp

the rutile and anatase single phase, respectively [41]. Peaks locatedat 235, 432 and 602 cm�1 can be assigned to the rutile phase,Fig. 3(a). No peaks that could be assigned to anatase TiO2 orbrookite were detected. In contrast, peaks located at 157, 392, 507and 628 cm�1, Fig. 3(b), can be assigned to the anatase phase, andno other peaks were observed. Peaks that could be assigned to thedoping oxides, such as Cr2O3, located at 550 cm�1 [42], were notfound. These results were consistent with the XRD results shownin Fig. 2.

Products prepared by ceramic method at high temperaturemust be milled to adjust the grain size of the pigments in func-tion of their applications. It is important to know the micro-structure and grain size of the samples to select one or otherapplication. For example, grain sizes of below one micron arerequired for inkjet applications [15]. Therefore, the average grainsize of the R2 and A1 samples were analysed by SEM. Micro-graphs and the grain size distribution of both powder samples areshown in Fig. 4. In both cases, there was no evidence of secondaryphases by EDX, and therefore, single-phase of Cr,SbeTiO2 solidsolutions were obtained. Spherical particles were observed andthe grain size distribution was around 600 nm in both samples.Shape of the particles is also important for inkjet processes.Particles with rounded shape, as obtained in this work, are moresuitable for inkjet fluid-dynamics instead of angular shapes,which are usually obtained from high energy ball milling. Pro-duction of inkjet inks involves a problem of the pigment sedi-mentation in the dispersant [15]. In this way, measurements of z-potential were performed in water with 0.1% of sodium hexam-etaphosphate (65e70%, Aldrich) [43], in order to determine theelectrostatic stabilization of the R2 and A1 samples. Values of thez-potential were �57 mV and �56.5 mV, respectively. Thesenegative values showed that the pigment can be dispersed,avoiding the sedimentation. z-potential values around 20 mV areobtained for the industrial pigment using glycol as dispersant[14]. Note that high z-potential values (positive or negative) arebetter to avoid sedimentation and therefore, to be applied ininkjet technology. But, other important properties such as surfacetension, density or viscosity of the ink should be consider beforethe application [11].

eratures: (a) 750, (b) 800, (c) 850 and (d) 900 �C.

M. Jovaní et al. / Dyes and Pigments 116 (2015) 106e113 111

3.2. Study of the anatase-rutile transformation and evaluation ofthe colouring performance of the pigment

Fig. 5 shows the evolution with temperature (from 750 to1080 �C) of the TiO2 polymorphs by Raman spectroscopy. Theanatase-rutile transformation occurred at temperatures higherthan 850 �C, and it was essentially completed at 1000 �C. Thechromatic coordinates of the samples at different temperatures areshown in Table 3 and Fig. 6. A1 sample presents a slight yellowcolouration at 180 �C. It is possible to relate the increase of a* and b*chromatic coordinates with the phase transitions between 750 �Cand 1080 �C. The b* coordinate increase from 23 to 36 suggesting

Fig. 8. Diffuse Reflectance spectra for R2 (a), A1 (b) and a commercial ceramic pigment(c) after glazing with deconvolution of optical bands.

that there is a relation between the polymorph transformation andthe increase of b* coordinate. It could be explain either by theincorporation of the Cr and Sb in different environments or, assuggest Matteucci et al. [36,44], because there is a change of the Sboxidation state. However, a* coordinate show an approximatelylinear increase with temperature over the range 750e1000 �C. It isimportant to highlight a constant b* value in the interval between900 and 1000 �C. In this range, the main polymorph is rutile butanatase phase exist as minor phase. These results suggest that thephase transformation is still going on at this range of temperatures,probably due to the necessary time for the reconstruction of thestructure, from anatase to rutile. After the reconstructive trans-formation was occurred, and single phase of rutile was obtained at1080 �C, the value of b* increase significantly.

In order to assess the morphology of the powders during thephase transformation, SEM analysis was made at each temperature.Fig. 7 shows the micrographs for the evolution of the morphologyat different temperatures. Shape of the particles was differentdepending on the polymorph. When anatase is presented, theparticles were almost spherical with certain agglomeration,Fig. 7(a) and (b). During the transition, when rutile phase was themajority phase, the particles were prismatic. This fact suggests theelongation of the particles in the polymorphic transition.

3.3. Stability of the particles as a pigment

In order to determine the stability of the powders prepared at180 �C after glazing, a micropowder/frit mixture was prepared andfired according the cycle set out in the experimental part at themaximum temperature of 1080 �C. The pieces with the glaze werealso characterized by UVeVis and values of the CIELab parameterswere obtained. Diffuse reflectance spectra of R2, A1 and a com-mercial ceramic pigment fired at higher temperatures (>1200 �C)and glazed in the same conditions are presented in Fig. 8 for the270e800 nm wavelength range. Three main absorption bands areidentified in the three spectra after the deconvolution: a broadband located at high energy and centred at ~340 nm (ultravioletregion) attributed to the bandgap of the rutile [45]; and two bandscentred at ~445 and 560 nm attributed to the ded transitions of theCr(III) [4A2(4F) / 4T1(4F) and 4A2(4F) / 4T2(4F), respectively] in anoctahedral coordination [39]. Higher absorption of the ded tran-sition 4A2(4F)/ 4T1(4F) was observed for the A1 glazed sample andthis intensity was similar to the glazed commercial pigment. Pre-cursor of titanium in solidesolid reactions is usually anatase, whichtransforms to rutile during the synthesis at high temperatures [38].The intensity of the absorption bands could be related with thechromatic coordinates shown in Table 4. Good chemical and ther-mal stability into the frit was obtained and pigments acquired or-ange colour after glazing with the frit for both samples. Chromaticcoordinates were slightly higher for the A1 and similar to thoseobtained for the commercial ceramic pigment in glaze (similar a*values). Photographs of the samples before and after mixed withthe frit are shown in Fig. 9. In summary, colour of both samples in astandard frit was similar to the colouration of the commercialceramic pigment obtained at high temperatures. Therefore,

Table 4CIELab parameters of R2 and A1 samples after glazing (frit). CIELab parameters of acommercial ceramic pigment are included as comparison.

Powder/Glaze 1080 �C

L* a* b*

R2 65 12 39A1 64 16 41Commercial (>1200 �C) 60 16 47

Fig. 9. Photographs of the ceramic tiles for R2 (a), A1 (b) and the commercial pigment (c) after mixed with the frit.

M. Jovaní et al. / Dyes and Pigments 116 (2015) 106e113112

synthesis route has produced orange pigments after glazing, similarof those commercial pigments. It is important to note that thesepigments with smaller particle size are stable in glazes, and theydeveloped similar colour of the commercial pigment.

This novel method to synthesize spherical submicron particlesat low temperature would presented benefits with respect to themethod used in the ceramic industry, which makes use of hightemperature calcination. Moreover, secondary phases are normallyobtained from traditional ceramic route, not allowing a control ofthe phase, shape and size. A milling stage must be implemented toadjust the grain size of the pigment due to the high particle sizecaused by high-temperature calcinations. For these reasons, thenovel synthesis method presented in this work to prepare pigmentsat low temperature would allowed several advantages compared tothe traditional solidesolid method, used in the ceramic industry. Apossible application of this method in the industry can avoid themilling stages and the calcinations at high temperature.

4. Conclusions

Anatase and rutile single phase of a yellow ceramic pigmentbased on Cr,SbeTiO2 were obtained by microemulsion mediatedsolvothermal method at 180 �C. The experimental conditions wereoptimised in order to obtain anatase or rutile phase. These samplesprepared at 180 �C were single phase by XRD, Raman spectroscopyand SEM/EDX. The solvothermal treatment time needed to obtainsingle phase of anatase or rutile in the Ti0.97Cr0.015Sb0.015O2 solidsolution was 17 h and 24 h, respectively. Spherical particles wereformed in both cases with an average particle size of 600 nm and az-potential value around �57 mV. The polymorphic transitiontemperature and the changes of the morphology that this trans-formation involves were determined by Raman and SEM. Theanatase-rutile transformation occurred at temperatures higherthan 850 �C, and it was essentially completed at 1000 �C. Elonga-tion of the particles in the polymorphic transition was observed.There is also a correlation between the polymorph transformationand the increase of the chromatic coordinates measured byUVeVis, leading to a huge orange colour pigment. Samples have agood chemical and thermal stabilization into the frit, presentingsimilar chromatic coordinates to those of the commercial ceramicpigment obtained at high temperatures, especially when theanatase solid solution at 180 �C was the starting powder. Orangecolour was kept after the application on glazes. Therefore, size,shape, colour and electrostatic stability of these particles, preparedby microemulsion-mediated solvothermal route, make it a poten-tial candidate for orange ceramic pigment to be incorporated in theinkjet technology.

Acknowledgements

We thank the “Universitat Jaume I”-project No. P11B2013-65 (MJ,MD, HB, EC) and the “INCTMN”-2008/57872-1, “FAPESP”-2013/

11144-3; 2013/07296-2, and “CNPq”573636/2008-7 (TRMandEL) forfinancial support. M.J. also thanks this project for a fellowship. Wealso thank to JuanCarlosGallard (Ferro Spain S.A.) for the initial ideas.

References

[1] Pfaff G, Reynders P. Angle-dependent optical effects deriving from submicronstructures of gilms and pigments. Chem Rev 1999;99:1963e82.

[2] Salvador A, Pascual-Martí MC, Adell JR, Requeni A, March JG. Analyticalmethodologies for atomic spectrometric determination of metallic oxides inUV sunscreen creams. J Pharm Biomed Anal 2000;22:301e6.

[3] Zallen R, Moret MP. The optical absorption edge of brookite TiO2. Solid StateCommun 2006;137:154e7.

[4] Braun JH, Baidins A, Marganski RE. TiO2 pigment technology: a review. ProgOrg Coatings Prog Org Coat 1992;20:105e38.

[5] Yuan S, Chen W, Hu S. Fabrication of TiO2 nanoparticles/surfactant polymercomplex film on glassy carbon electrode and its application to sensing tracedopamine. Mater Sci Eng C 2005;25:479e85.

[6] Mo S, Ching Y. Electronic and optical properties of three phases of titaniumdioxide: rutile, anatase, and brookite. Phys Rev 1995;51:23e32.

[7] Hanaor DH, Sorrell CC. Review of the anatase to rutile phase transformation.J Mater Sci 2010;46:855e74.

[8] Braginsky L, Shklover V. Light absorption in TiO2 nanoparticles. Eur Phys JD1999;630:627e30.

[9] BeydounD,AmalR. Implicationsofheat treatmenton thepropertiesofamagneticiron oxide-titanium dioxide photocatalyst. Mater Sci Eng B 2002;94:71e81.

[10] Chen X, Mao SS. Titanium dioxide nanomaterials: synthesis, properties,modifications, and applications. Chem Rev 2007;107:2891e959.

[11] Reidy DJ, Holmes JD, Morris MA. The critical size mechanism for the anatase torutile transformation inTiO2 anddoped-TiO2. J Eur CeramSoc 2006;26:1527e34.

[12] Yin S, Hasegawa H,Maeda D, IshitsukaM, Sato T. Synthesis of visible-light-activenanosize rutile titania photocatalyst by low temperature dis-solutionereprecipitation process. J Photochem Photobiol A Chem2004;163:1e8.

[13] Li G, Boerio-Goates J, Woodfield BF, Li L. Evidence of linear lattice expansionand covalency enhancement in rutile TiO2 nanocrystals. Appl Phys Lett2004;85:2059e61.

[14] Gardini D, Dondi M, Costa AL, Matteucci F, Blosi M, Galassi C, et al. Nano-sizedceramics inks for drop-on-demand ink-jet printing in quadrichromy. J NanosciNanotechnol 2008;8:1979e88.

[15] Dondi M, Blosi M, Gardini D, Zanelli C. Ceramic pigments for digital decorationinks: an overview. Castell�on: Qualicer; 2012. p. 1e11.

[16] Kosmala A, Wright R, Zhang Q, Kirby P. Synthesis of silver nano particles andfabrication of aqueous Ag inks for inkjet printing. Mater Chem Phys 2011;129:1075e80.

[17] Feldmann C, Jungk HO. Polyol-mediated preparation of nanoscale oxide par-ticles. Angew Chem Int Ed 2001;40:359e62.

[18] Baldi G, Bitossi M, Barzanti A. US patent 2008; 7316741 B2.[19] Cavalcante PMT, Dondi M, Guarini G, Raimondo M, Baldi G. Colour perfor-

mance of ceramic nano-pigments. Dye Pigment 2009;80:226e32.[20] Zhong Z, Ang TP, Luo J, Gan H, Gedanken A. Synthesis of one-dimensional and

porous TiO2 nanostructures by controlled hydrolysis of titanium alkoxide viacoupling with an esterification reaction. Chem Mater 2005;17:6814e8.

[21] Song KC, Pratsinis SE. Control of phase and pore structure of titania powdersusing HCl and NH4OH catalysts. J Am Ceram Soc 2001;84:92e8.

[22] Tayade RJ, Kulkarni RG, Jasra RV. Photocatalytic degradation of aqueousnitrobenzene by nanocrystalline TiO2. Ind Eng Chem Res 2006;45:922e7.

[23] Murakami Y, Matsumoto T, Takasu Y. Salt catalysts containing basic anionsand acidic cations for the sol-gel process of titanium alkoxide: controlling thekinetics and dimensionality of the resultant titanium oxide. J Phys Rev B1999;103:1836e40.

[24] Cheng H, Ma J, Zhao Z, Qi L. Hydrothermal preparation of uniform nanosizerutile and anatase particles. Chem Mater 1995;7:663e71.

[25] Li G, Gray KA. Preparation of mixed-phase titanium dioxide nanocompositesvia solvothermal processing. Chem Mater 2007;19:1143e6.

[26] Lal M, Chhabra V. Preparation and characterization of ultrafine TiO2 particlesin reverse micelles by hydrolysis of titanium di-ethylhexyl sulfosuccinate.J Mater Res 1998;13:1249e54.

M. Jovaní et al. / Dyes and Pigments 116 (2015) 106e113 113

[27] Andersson M, Lars O. Preparation of nanosize anatase and rutile TiO2 by hy-drothermal treatment of microemulsions and their activity for photocatalyticwet oxidation of Phenol. J Phys Chem B 2002;106:10674e9.

[28] Deorsola FA, Vallauri D. Study of the process parameters in the synthesis ofTiO2 nanospheres through reactive microemulsion precipitation. PowderTechnol 2009;190:304e9.

[29] Cao M, Wu X, He X, Hu C. Microemulsion-mediated solvothermal synthesis ofSrCO3 nanostructures. Langmuir 2005;21:6093e6.

[30] Cao M, Wang Y, Guo C, Qi Y, Hu C. Preparation of ultrahigh-aspect-ratio hy-droxyapatite nanofibers in reverse micelles under hydrothermal conditions.Langmuir 2004;20:4784e6.

[31] Wu M, Long J, Huang A, Luo Y. Microemulsion-mediated hydrothermal syn-thesis and characterization of nanosize rutile and anatase particles. Langmuir1999;15:8822e5.

[32] Lu CH, Wu WH, Kale RB. Microemulsion-mediated hydrothermal synthesis ofphotocatalytic TiO2 powders. J Hazard Mater 2008;154:649e54.

[33] Shen X, Zhang J, Tian B. Microemulsion-mediated solvothermal synthesis andphotocatalytic properties of crystalline titania with controllable phases ofanatase and rutile. J Hazard Mater 2011;192:651e7.

[34] Aruna ST, Tirosh S, Zaban A. Nanosize rutile titania particle synthesis via ahydrothermal method without mineralizers. J Mater Chem 2000;10:2388e91.

[35] S�omiya S, Roy R. Hydrothermal synthesis of fine oxide powders. Bull Mater Sci2000;23:453e60.

[36] Matteucci F, Cruciani G, Dondi M, Raimondo M. The role of counterions (Mo,Nb, Sb, W) in Cr-, Mn-, Ni- and V-doped rutile ceramic pigments. Ceram Int2006;32:385e92.

[37] Batzill M, Morales EH, Diebold U. Influence of nitrogen doping on the defectformation and surface properties of TiO2 rutile and anatase. Phys Rev Lett2006;96:026103. 1-4.

[38] Gargori C, Cerro S, Galindo R, Monr�os G. In situ synthesis of orange rutileceramic pigments by non-conventional methods. Ceram Int 2010;36:23e31.

[39] Cruciani G, Dalconi MC, Dondi M, Meneghini C, Matteucci F, Barzanti A, et al.Temperature-resolved synchrotron X-ray diffraction of nanocrystalline titaniain solvent: the effect of CreSb and VeSb doping. J Nanopart Res 2010;13:711e9.

[40] Marfunin S. Physics of minerals and inorganic materials. Berlin, Heildelberg,New York: Springer; 1979.

[41] Chang H, Huang PJ. Thermo-raman studies on anatase and rutile. J RamanSpectrosc 1998;29:97e102.

[42] Sammelselg V, Tarre A, Lu J, Aarik J, Niilisk A, Uustare T, et al. Structuralcharacterization of TiO2eCr2O3 nanolaminates grown by atomic layer depo-sition. Surf Coatings Technol 2010;204:2015e8.

[43] Wakeman RJ, Marchant JQ. The influence of pH and temperature on therheology and stability of aqueous titanium dioxide dispersions. Chem Eng J1997;67:97e102.

[44] Dondi M, Cruciani G, Guarini G, Matteucci F, Raimondo M. The role of coun-terions (Mo, Nb, Sb, W) in Cr-, Mn-, Ni- and V-doped rutile ceramic pigments.Ceram Int 2006;32:393e405.

[45] Yan J, Wu G, Guan N, Li L, Li Z, Cao X. Understanding the effect of surface/bulkdefects on the photocatalytic activity of TiO2: anatase versus rutile. Phys ChemChem Phys 2013;15:10978e88.